Change the doppler convention

[astrofle]

It must be possible to change the Doppler convention used between "radio", "optical" and "relativistic".

specutils.Spectrum1D provides a with_velocity_convention() method that can handle this, however, it conflicts with the current definition of the dysh.spectra.Spectrum class. An example follows.
The solution does not need to use specutils.Spectrum1D.with_velocity_convention, but it would be good if dysh does not break its functionality.

from dysh.fits.gbtfitsload import GBTFITSLoad
from astropy.utils.data import download_file

raw_data = "http://www.gb.nrao.edu/dysh/example_data/onoff-L/data/TGBT21A_501_11.raw.vegas.fits"
raw_data_path = download_file(raw_data)
sdf = GBTFITSLoad(raw_data_path)
tp = sdf.gettp(152, ifnum=0, plnum=0)
ta = tp.timeaverage()
ta[0].with_velocity_convention("radio")

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[44], line 1
----> 1 ta[0].with_velocity_convention("radio")

File /home/sandboxes/psalas/Dysh/py3.11-dysh-note/lib/python3.11/site-packages/specutils/spectra/spectrum_mixin.py:132, in OneDSpectrumMixin.with_velocity_convention(self, velocity_convention)
    131 def with_velocity_convention(self, velocity_convention):
--> 132     return self.__class__(flux=self.flux, wcs=self.wcs, meta=self.meta,
    133                           velocity_convention=velocity_convention)

File /home/sandboxes/psalas/Dysh/dysh/src/dysh/spectra/spectrum.py:61, in Spectrum.__init__(self, *args, **kwargs)
     59     self._obstime = None
     60 if "CTYPE1" in self.meta:
---> 61     self._target.topocentric = is_topocentric(self.meta["CTYPE1"])
     62     self.topocentric = is_topocentric(self.meta["CTYPE1"])
     64 # if mask is not set via the flux input (NaNs in flux or flux.mask),
     65 # then set the mask to all False == good

AttributeError: 'NoneType' object has no attribute 'topocentric'